The concept of digital networks such as Integrated Service Digital Network (I.S.D.N.), began in the early sixties, with the process of digitalization of telephone networks in different countries, thus replacing the earlier analog technologies.
The concept of I.S.D.N. involving fully digital connections between data communication equipments, still far from complete, is expected to offer substantial advantages such as an economy of network costs, an economy in the use of the equipment, a flexibility of planning, less risk generated by the uncertainty of the demand for new services, uniform procedures for access to networks and relative simplification for users, considerable improvement in quality.
I.S.D.N. standards defined and studied by CCITT recommendations provide principles and guidelines on the I.S.D.N. concept, and detailed specifications of the user-network and internetwork interfaces.
FIG. 1 illustrates a simplified CCITT architectural reference model which is essential to the understanding of the structure of the physical reference points for the different I.S.D.N. interfaces (R, S, T, U and V).
This model provides that the equipments available to the users, namely TE1 and TE2, are connected to functional units, such as NT2 itself connected to functional unit NT1, via, when necessary, a terminal adapter (TA). Multiplexing operations as well as the level adaptation of the signal existing on the lines to the network on one hand, and signalling and maintenance operations on the other hand, are performed in functional units NT1/NT2.
Network Termination equipment NT2 allows the connection of data termination equipments and may also provide services to the subscriber without involving the network. This may be the case when using private switching systems of the PABX type, or a local LAN network. However, Network Terminating equipment NT2 can also be reduced to a "passive bus", i.e. a parallel connection on a cable, the subscriber line terminal (NT1 and NT2) being thus called "transparent".
Network Termination equipment NT1 performs the ISO layer 1 conversion from a two wire U interface, active on the line between the local exchange and the subscriber, and a two wire T interface. Basically, Network Termination equipment NT1 provides baseband conversion functions and network termination functions such as power supply, maintenance. . . .
Interfaces S and T of Network Termination equipments NT1 and NT2, described in CCITT I.430 recommendations, are the basic interfaces of any equipments designed to be connected to the I.S.D.N. The S interface provides a universal terminal interface, for the telephony as will for teleinformatics; it is most likely to supersede all specific data communication existing interfaces such as V24 (for the Data Terminating equipment of the future called TEl), at the end of a long process of transition during which Terminal Adapters TA are needed in order to connect an existing data termination equipment TE2 to I.S.D.N. (i.e. synchronous and asynchronous V.series, X21 and X21bis, X25 . . . ). The T interface materializes the only access to the network. Different standards have been defined, which correspond to different equipment sizes. R interface reference point is used to connect existing terminal equipment (X.21, V.24/V28. etc.) which are pre-I.S.D.N. terminals, to an I.S.D.N. network via a terminal adapter TA. U interface will be available for attachment in USA. V interface is the I.S.D.N. internal network interface header for the user.
Recommendation 1.430 defines layer characteristics of the user-network interface to be applied at the S or T reference points for the basic interface structure. I.S.D.N. basic access provides a 144 kb/s transmission channel which is divided in three subchannels by means of a Time Division Multiplex mechanism: two B channels and a single D channel. Each of the B-channels is a 64 kbit/s duplex bearer channel with bit and octet timing which can be used for a digital telephone communication (the Pulse Code Modulation PCM is performed in the subscriber equipment) or for a 64 kbit/s data communication. The D-channel is a 16 kbit/s channel allowing dynamic multiplexing of the following information: signalling information relative to the B-channels, information relative to low rate teleservices, and information using low rate Packet Switching services (till 9 600 bit/s and even 16 kbit/s).
As seen previously, the connection of an existing terminal equipment TE2, of the type of the V. or X. type in the Recommendations will need a terminal adapter TA.
This Terminal Adapter TA should support various asynchronous and synchronous user data rates including at least the recommended basic set of speeds. To achieve this goal, Terminal Adapter TA must be able to perform multiplexing, rate adaptation and support of existing interfaces. According to CCITT I.460 Recommendation, Terminal Adapter should be able to adapt the rate of one stream, having a rate lower than 64 kbit/s, into a 64 kbit/s B-channel and to multiplex several streams, having rates lower than 64 kbit/s, into a single 64 kbit/s channel. The rates lower than 64 kbit/s may be of two types: binary rates of 8, 16 and 32 kbit/s and other rates including those associated with Data Terminating Equipment DTE conforming to the X and V Recommendations. Therefore, a rate adaptation system allowing the conversion of bit rates and facilitating the multiplexing of different streams into a B or D channel is needed. Since the concept of I.S.D.N. will provide in every customer's home the possibility of connection to the future digital network and thus will allow the customer to access large public telecommunications services, telecommunication product suppliers will have to market in large quantities terminal adapters TA designed to allow the connection of a wide range of existing data terminal equipments TE2 having different interfaces according to the V. or X. Recommendations. Terminal Adapter TA should use a simple and low cost rate adaptation system allowing the conversion of a stream having different possible rates lower than 64 kbit/s into a 64 kbit/s stream. More particularly, this rate adaptation device should involve few components in order to limit the manufacturing costs while assuring an adaptability of the terminal adapter to a large number of different existing Data Terminating Equipment DTE having different interfaces of the V. or the X. Recommendations. Moreover, the latter rate adaptation system should facilitate the further multiplexing of different streams in order to achieve a good utilization of the bandwidth provided by a B or D channel.